Endophytic entomopathogenic fungi promote growth and prime phenylpropanoid defenses for enhanced insect resistance in willows

The entomopathogenic fungus Beauveria bassiana directly infects pests and acts as an endophyte in herbaceous plants. However, its capacity to endophytically colonize woody plants and the underlying mechanisms of induced insect resistance remain largely unexplored. This study investigated B. bassiana endophytic colonization of Salix matsudana via foliar spray, confirming successful early (7 dpi) and late (21 dpi) stage colonization, despite a declining rate over time. Late-stage colonized plants exhibited enhanced growth and stronger defense, leading to significant inhibition of leaf beetle survival and development. This enhanced toxicity was evidenced by dual suppression of the beetle’s key immunity and detoxification gene expression. Further transcriptomic and metabolomic analyses revealed that late-stage endophytic colonization by B. bassiana primed the plant’s defense by upregulating key genes and accumulating functional metabolites in the phenylpropanoid biosynthesis pathway. Key pathway genes were upregulated and remained highly expressed after herbivory, with associated metabolites like trans-cinnamic acid. Functional validation confirmed that these metabolites significantly altered beetle feeding preference, inhibited larval weight gain and pupation, and increased mortality. This study elucidates the molecular ecological mechanism of fungus-induced insect resistance in a woody plant, establishing a theoretical basis for plant-endophyte-herbivore interactions and offering a valuable reference for herbivore -mediated plant defense. • The entomopathogenic fungus Beauveria bassiana endophytically colonizes the woody plant Salix matsudana . • B. bassiana colonization promotes willow growth, primes plant defense, and enhances resistance against leaf beetles. • Late-stage B. bassiana colonization upregulates phenylpropanoid pathway genes and induces the accumulation of functional metabolites. • Induced metabolites by B. bassiana colonization alter beetle feeding preference, inhibit larval development, and increase mortality.